The present invention relates to an audio signal processing device for performing various kinds of processing on an audio signal, and more particularly, to a technique for expanding the low-frequency components of the signal.
It is physically impossible for speakers with a small diameter to output the low-frequency components of an audio signal in some cases. A sub-woofer is generally used to complement the low-frequency components. However, such a sub-woofer is generally a monaural speaker and cannot properly output source sound of a plurality of channels. For example, even in the case that a drum is placed at the left channel and a bass is placed at the right channel, the sounds thereof are eventually output from a single sub-woofer.
Hence, a method has been proposed in which it is perceived as if low-frequency components, not output physically, are output by utilizing a phenomenon referred to as “missing fundamental” produced by generating the harmonics of the low-frequency components of an audio signal and by adding the harmonics to the original audio signal.
For example, the device disclosed in JP-A-2011-87192 performs power calculations (for example, third and fourth power calculations) of an audio signal and generates odd-numbered and even-numbered order harmonics.
In the case that a power calculation (for example, a second power calculation) is performed for an original signal having negative components as shown in FIG. 10A, the negative components are converted into positive components (full-wave rectified) as shown in FIG. 10B, whereby the waveform of the original signal cannot be maintained. In particular, as indicated by broken lines in FIG. 10B, at positions in which the amplitude value of the signal is zero, the waveform becomes non-linear after the second power calculation although the original signal had a linearly zero-crossing waveform, thereby causing unwanted distortion components. As a result, for example, in a steadily generated bass sound having little distortion (for example, the bass of a stringed instrument), a distortion due to this kind of change in the waveform becomes conspicuous.